1. Field of the Invention
This invention relates to the automatic control of both feed to a gypsum calcining kettle and discharge of calcined gypsum from the kettle. More specifically, the invention relates to controlling the feed to the kettle by the temperature of the finished product and discharge of the finished product from the kettle, at a rate maintaining a predetermined level of the gypsum in the kettle.
2. Prior Art
The gypsum wallboard industry is energy intensive. It is well established that 3,000 BTU per square foot of wallboard is required. Many plants with a speed of 150 fpm manufacture approximately 36,000 square feet per hour, which requires an energy consumption up to 108,000,000 BTU/hr. About two-thirds of the heat requirement is used in the wallboard plant, the other one-third (36,000,000 BTU/hr.) is used in the gypsum calcining (stucco or plaster) mill.
It is common practice to calcine gypsum on a batch basis. Despite the general desire to automate kettle operation, there is great inertia in the industry against the changes necessary to automate the production of the calcined product.
There have been systems proposed for automation which have been quite elaborate and expensive. These systems have all included the concept of discharging the calcined product from the kettle bottom through a standpipe, or syphon with its attendant disadvantages. The drawbacks to the so-called tea kettle spout, or syphon, are the need to add extra heat to the spout, and also the need for compressed air jets in the bottom of the kettle to start the product flowing. This has led to plugged up-take pipes, which has aborted the start-up of the continuous kettle.
What is needed is a coordinated control of the feed and discharge from the kettle to produce the product at the proper temperature from the discharge, and to maintain the level of material in the kettle within a range of predetermined values. Further, the discharge in the kettle should be positioned at the same level as the batch operation discharge, and be regulated by a valve responsive to the control factors of kettle operation.